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| <title>Using the GNU Compiler Collection (GCC): Local Reg Vars</title> |
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| Previous: <a href="Global-Reg-Vars.html#Global-Reg-Vars" accesskey="p" rel="prev">Global Reg Vars</a>, Up: <a href="Explicit-Reg-Vars.html#Explicit-Reg-Vars" accesskey="u" rel="up">Explicit Reg Vars</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Specifying-Registers-for-Local-Variables"></a> |
| <h4 class="subsubsection">6.43.5.2 Specifying Registers for Local Variables</h4> |
| <a name="index-local-variables_002c-specifying-registers"></a> |
| <a name="index-specifying-registers-for-local-variables"></a> |
| <a name="index-registers-for-local-variables"></a> |
| |
| <p>You can define a local register variable with a specified register |
| like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">register int *foo asm ("a5"); |
| </pre></div> |
| |
| <p>Here <code>a5</code> is the name of the register that should be used. Note |
| that this is the same syntax used for defining global register |
| variables, but for a local variable it appears within a function. |
| </p> |
| <p>Naturally the register name is CPU-dependent, but this is not a |
| problem, since specific registers are most often useful with explicit |
| assembler instructions (see <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>). Both of these things |
| generally require that you conditionalize your program according to |
| CPU type. |
| </p> |
| <p>In addition, operating systems on one type of CPU may differ in how they |
| name the registers; then you need additional conditionals. For |
| example, some 68000 operating systems call this register <code>%a5</code>. |
| </p> |
| <p>Defining such a register variable does not reserve the register; it |
| remains available for other uses in places where flow control determines |
| the variable’s value is not live. |
| </p> |
| <p>This option does not guarantee that GCC generates code that has |
| this variable in the register you specify at all times. You may not |
| code an explicit reference to this register in the assembler |
| instruction template part of an <code>asm</code> statement and assume it |
| always refers to this variable. |
| However, using the variable as an input or output operand to the <code>asm</code> |
| guarantees that the specified register is used for that operand. |
| See <a href="Extended-Asm.html#Extended-Asm">Extended Asm</a>, for more information. |
| </p> |
| <p>Stores into local register variables may be deleted when they appear to be dead |
| according to dataflow analysis. References to local register variables may |
| be deleted or moved or simplified. |
| </p> |
| <p>As with global register variables, it is recommended that you choose a |
| register that is normally saved and restored by function calls on |
| your machine, so that library routines will not clobber it. |
| </p> |
| <p>Sometimes when writing inline <code>asm</code> code, you need to make an operand be a |
| specific register, but there’s no matching constraint letter for that |
| register. To force the operand into that register, create a local variable |
| and specify the register in the variable’s declaration. Then use the local |
| variable for the asm operand and specify any constraint letter that matches |
| the register: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">register int *p1 asm ("r0") = …; |
| register int *p2 asm ("r1") = …; |
| register int *result asm ("r0"); |
| asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2)); |
| </pre></div> |
| |
| <p><em>Warning:</em> In the above example, be aware that a register (for example r0) can be |
| call-clobbered by subsequent code, including function calls and library calls |
| for arithmetic operators on other variables (for example the initialization |
| of p2). In this case, use temporary variables for expressions between the |
| register assignments: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">int t1 = …; |
| register int *p1 asm ("r0") = …; |
| register int *p2 asm ("r1") = t1; |
| register int *result asm ("r0"); |
| asm ("sysint" : "=r" (result) : "0" (p1), "r" (p2)); |
| </pre></div> |
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